Method of growing silicon crystal in liquid phase and method of producing solar cell

ABSTRACT

The method of the present invention of growing single crystal silicon in a liquid phase comprises preparing a melt by dissolving a solid of silicon containing boron, aluminum, phosphorus or arsenic at a predetermined concentration into indium melted in a carbon boat or a quartz crucible, supersaturating the melt, and submerging a substrate into the melt, thereby growing a silicon crystal containing a dopant element. This method can provide a method of growing a thin film of crystalline silicon having a high crystallinity and a dopant concentration favorably controlled, thereby serving for mass production of inexpensive solar cells which have high performance as well as image displays which have high contrast and are free from color ununiformity.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of growing a siliconcrystal in a liquid phase. A silicon crystal produced by the method ofthe present invention can be used in silicon devices having a large areasuch as solar cells and picture element driving circuits for liquidcrystal display devices.

[0003] 2. Related Background Art

[0004] Solar cells are prevailing as electric power sources which aresystematically linked with driving power sources for various kinds ofappliances and commercial line power. It is desirable that solar cellscan be manufactured at low costs. For example, it is desired to producesolar cells on inexpensive substrates at a low cost. Silicon isgenerally used as a semiconductor for composing solar cells. Singlecrystalline silicon is extremely excellent from a viewpoint ofefficiency of converting a light energy into electric power, that is,photoelectric conversion efficiency. From a viewpoints of enlargement ofareas and reduction of manufacturing costs, on the other hand, amorphoussilicon is advantageous. In the recent years, it has come to usepolycrystalline silicon for the purpose of obtaining a cost as low asthat of amorphous silicon and a photoelectric conversion efficiency ashigh as that of the single crystalline silicon.

[0005] However, it cannot be said that the expensive crystallinematerials are sufficiently utilized by a method which is conventionallyadopted to manufacture silicon devices using single crystalline siliconor polycrystalline silicon since the method is configured to slice alump crystal to form plate-like substrates and hardly capable ofpreparing substrates which have thickness of 0.3 mm or smaller, therebyallowing the substrates to have thicknesses larger than a thickness (20μm to 50 μm) generally required to absorb incident rays. Furthermore,there has recently been proposed the spin method of forming a siliconsheet by flowing drops of melted silicon into a template. However, asilicon sheet formed by this method has a quality insufficient for useas a semiconductor and cannot provide a photoelectric conversionefficiency which is so high as that in the case of using a generalcrystalline silicon.

[0006] There has been proposed and actually applied to trial productionof a solar cell under the circumstances described above, an idea ofgrowing on an inexpensive substrate a silicon crystal of a good qualityuntil it has a required and sufficient thickness, and form an activeregion (for example, a photoelectric conversion region) thereon.Moreover, there has been proposed an idea of growing a silicon crystalepitaxially on a substrate of a good quality and then peel off thesilicon crystal and reuse the substrate.

[0007] On a premise that large area devices such as solar cells are tobe produced in mass, however, it is not so easy to grow a siliconcrystal until it has a thickness required for absorbing incident rays. Asilicon crystal of a good quality is generally grown by the thermal CVDmethod of thermally decomposing a raw material gas such as silanechloride. In order to grow a single crystal at a high rate on the orderof 1 μm/minute in particular, it is general to use the so-calledepitaxial growing furnace. However, such a growing furnace is not onlyunsuited to mass production since it can treat 10 wafers at most at onebatch but also requires a high raw material cost since it utilizes a rawmaterial gas at a low efficiency. Though it is possible to treat 100 ormore wafers at one batch by utilizing the so-called low pressure CVDfurnace, this furnace also provides a crystal insufficient in a qualitythereof and allows the crystal to grow at a rate only on the order of0.01 μm/minute, thereby being low in productivity.

[0008] As another method of growing a silicon crystal, there is known aliquid phase growing method of supersaturating a liquid metal solutionin which silicon is dissolved and allowing a crystal to deposit from thesolution onto a substrate. This liquid phase growing method is capableof growing a crystal of a high quality at a high rate on the order of 1μm/minute and treating 100 or more wafers at one batch, thereby beingsuited to mass production. However, the liquid phase growing method isnot generally used for growing silicon and has some technical problemsto be solved though it widely prevails as a method of growing compoundsemiconductors.

[0009] One of important problems lies in selection of a metal which isto be used as a solvent. It is desirable that a metal to be used forthis purpose has a solubility for silicon which is as high as possibleand can hardly be incorporated into deposited silicon. Furthermore, ametal having a lower melting point and a lower vapor pressure can behandled easier. Tin is used most generally as a solvent for silicon. Tincan be handled relatively easily since it has allow melting point and arelatively high solubility for silicon. It has been considered that tinis a preferable solvent since tin and silicon belong to the same IVgroup of the Periodic Table, and tin is inactive as a dopant even whenit is incorporated into deposited silicon.

[0010] However, the inventors have recently found that tin isincorporated into silicon in a prettily large amount when growthconditions (in particular, a growth temperature) are inadequate, therebydeforming a lattice of a silicon crystal and adversely affectingelectric characteristics of a semiconductor probably due to the atomicsize of tin very different from that of silicon though they are atomsbelonging to the IV group. From this viewpoint, there is posed a doubtin aptitude of tin as a solvent which is used to grow a crystal for asolar cell with high efficiency.

[0011] In addition to tin, elements such as gallium, indium and aluminumwhich belong to the III group can be mentioned as metals which areusable as solvents. Gallium and indium, in particular, having a lowmelting point can be handled easily. Since gallium is extremelyexpensive, indium is hopeful for use as a practical melt. However,indium posed a problem which is described later in controlling byintroducing dopant a conductivity type of a silicon crystal which isgrown using an indium melt. There are known examples wherein gallium isused as p-type dopant in combination with an indium melt (G. F. Zheng etal.: Solar Energy Materials and Solar Cells. 40 (1996) 231-238). Thoughgallium is usable at relatively low concentrations, it cannot be usedfor doping at high concentrations since a solid of gallium can bedissolved into silicon at concentrations within a relatively lowsolubility and is extremely expensive. On the other hand, examples whichuse n-type dopants in combination with indium melts are disclosed byJapanese Patent Application Laid-Open Nos. 9-183695 and 9-183696.

[0012] Boron and aluminum are generally used as p-type dopants, whereasphosphorus and arsenic are often used as n-type dopants. It is thereforeconceivable to use these dopants for growing silicon crystals in liquidphase with the indium melt. In practice, however, problems were posed inconductivity types or reproducibility of conductivities of grown siliconcrystals in certain cases. Furthermore, it is feared that a metal of theIII group such as indium which is originally active by itself as adopant may control a crystal to a strong p-type when incorporated intosilicon and may be incapable of controlling it to p⁻-type or n-type.

[0013] The problems described above makes it still impossible to judgewhether or not the liquid phase method has a true aptitude for growth ofsilicon crystals on scales of mass production and whether or not solarcells utilizing thin films of silicon crystals have practical utility.

[0014] Thin films of silicon crystals are also used as devices fordriving picture elements of liquid crystal displays and so on.Progresses made in the mass communication media have produced increasingdemands for a display having a larger screen and capable of moreminutely driving at a higher speed. Though the TFTs (thin filmtransistors) of amorphous silicon have hitherto been utilized as adriving circuit for picture elements to cope with the demands for adisplay having a larger screen, the amorphous silicon cannot meet anylonger the demands for a display which can be more minutely driven at ahigher speed, and it is becoming to use TFTs of polycrystalline silicon.In addition, there has been increasing demands for polycrystallinesilicon which has higher carrier mobility and other characteristics.

[0015] The liquid phase growing method is also suited for growing suchcrystalline silicon of a high quality on a large substrate such as aglass plate. Though use of a glass plate or the like makes itunallowable to heat a solution to a high temperature, it is possible togrow a crystal of a good quality by using indium as a solvent. Though itis impossible to grow a thick crystal at a low growth temperature whichlowers a solubility of silicon into indium, there is no problem information of a crystal to be used as a TFT having a thickness of theorder of 0.1 to 0.5 μm which is far smaller than that of a solar cell.When indium is used as a solvent for production of a TFT, a problemrelated to reproducibility may be posed. Therefore, a concentration of adopant must be precisely controlled in order to enhance reproducibilityof characteristics of the TFT. In formation of a film having a largearea, an ununiform distribution of a dopant concentration is notpreferable which produces an ununiform distribution of characteristicsof TFT, thereby producing variations in image density on a displaydevice. In certain cases where indium was used as a dopant, it wasimpossible to sufficiently prevent the dopant from being distributedununiformly on surfaces.

SUMMARY OF THE INVENTION

[0016] The present invention has been achieved in view of the currentcircumstances described above, and an object of the present invention isto provide a method of precisely controlling a dopant to be incorporatedinto crystalline silicon which is grown in a liquid phase using indiumas a solvent, thereby enabling mass production of solar cells having ahigh efficiency and a light weight as well as driving circuits for ahigh precision and high speed display having a large area.

[0017] The present invention therefore provides a method of growing asilicon crystal, which comprises using a melt prepared by dissolving asolid of silicon containing a dopant at a predetermined concentrationinto liquid indium. Furthermore, the present invention provides a methodof growing a silicon crystal, which comprises using a melt prepared bydissolving a solid of indium containing a dopant at a predeterminedconcentration into liquid indium.

[0018] Moreover, the present invention provides a method of producing asolar cell, which comprises the steps: preparing a melt by dissolving asolid of silicon containing a dopant at a predetermined concentrationinto liquid indium; forming a first silicon layer of a firstconductivity type on a substrate by bringing the substrate into contactwith the melt; and forming a second silicon layer of a secondconductivity type on the first silicon layer of the first conductivitytype.

[0019] In addition, the present invention provides a method of producinga solar cell, which comprises the steps of: preparing a melt bydissolving a solid of indium containing a dopant at a predeterminedconcentration into liquid indium and further dissolving silicon into themelt; forming a first silicon layer of a first conductivity type on asubstrate by bringing the substrate into contact with the melt; andforming a second silicon layer of a second conductivity type on thefirst silicon layer of the first conductivity type.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a sectional view for showing one example of a solar cellaccording to the present invention;

[0021]FIGS. 2A, 2B and 2C are sectional views for showing one example ofproduction steps of a solar cell according to the present invention;

[0022]FIG. 3 is a sectional view for showing one example of an apparatusused for carrying out a method of producing a silicon crystal accordingto the present invention;

[0023]FIG. 4 is a sectional view for showing one example of an apparatusused for carrying out the method of producing a silicon crystalaccording to the present invention;

[0024]FIG. 5 is a sectional view for showing one example of an apparatusused for carrying out the method of producing a silicon crystalaccording to the present invention; and

[0025]FIGS. 6A, 6B, 6C, 6D, 6E and 6F are sectional views for showingone example of production steps of a thin film transistor (TFT) ofpolycrystalline silicon to which the method of the present invention isapplied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The present invention has been achieved on the basis of knowledgeobtained by experiments which are described below.

[0027] First, commercially available indium pellets were put into acarbon crucible, heated and melted at 1000° C. in a hydrogen gas flow toobtain liquid indium. A melt was prepared by bringing non-dopedpolycrystalline silicon into contact with the liquid indium anddissolving silicon into the liquid indium until it was saturated. Then,the melt was gradually cooled until it was supersaturated. When the meltwas cooled to 980° C., a substrate of non-doped polycrystalline siliconwas brought into contact with the melt, whereby a silicon crystal havinga thickness of 10 μm was epitaxially grown on the substrate. Ameasurement of specific resistance of the silicon crystal by thefour-probe method indicated approximately 0.2 Ωcm. Specific resistancewas varied within a range from 0.1 to 0.5 Ωcm in similar experimentswhich were carried out using three different lots of commerciallyavailable indium as the melt.

[0028] A similar experiment was carried out using indium pellets refinedto a high purity (6N), whereby a grown silicon layer on the substratehas an extremely high resistance (a difference in resistance between thegrown silicon and the substrate could not be evaluated). By secondaryion mass spectrometry (SIMS) analysis of impurities contained in thegrown silicon layer, no indium itself was unanticipatedly detected inany sample (below measurable limit). However, it was found that variouskinds of impure elements such as gallium and aluminum of the III groupin particular other than indium were contained in samples which weregrown using commercially available indium. From this result, it ispresumed that indium itself can hardly be incorporated into a siliconcrystal grown in a liquid phase, but the elements of the III group otherthan indium which were contained in the commercially available indiumpellets were easily incorporated into the silicon crystals, therebylowering resistance. In other words, it is necessary for precise controlof conductivity of silicon to precisely control impurities, inparticular, elements of the III group which are contained in indium.

[0029] Then, silicon was grown using a melt which was prepared bydissolving silicon into highly pure indium until it was saturated andthen dissolving pellets of boron and aluminum. However, specificresistance of silicon grown as described above was low inreproducibility. The result of SIMS analysis indicated variations inconcentrations of boron and aluminum in the silicon. Furthermore,silicon was grown using a melt which was prepared by dissolving siliconinto highly pure indium until it was saturated, and then dissolvingpowders of phosphorus and arsenic. The silicon grown as described abovewas certainly of the n-type but its specific resistance was low inreproducibility. The result of SIMS analysis indicated variations inconcentrations of phosphorus and arsenic in the silicon.

[0030] The inventors considered a cause for these variations asdescribed below. Since boron (density=2.23), aluminum (density=2.70),phosphorus (density=2.69) and arsenic (density=3.9) which are used asthe dopants are prettily lighter than indium (density=7.28), a solutionof the dopant tends to be concentrated on a surface of the indium melt,whereby the indium solution can hardly be uniform as a whole.Furthermore, solubilities of impurities which are subsequently dissolvedinto indium are influenced at a high possibility by a concentration ofsilicon which has already been dissolved in indium. In particular, whenindium is nearly saturated with silicon, a slight variation in thesaturation remarkably produces influences on the solubilities of theimpurities, whereby the elements which are put into the melt asimpurities are not always dissolved actually and concentrations of theelements of the impurities may be unstable in the melt.

[0031] It is possible to dissolve the elements of the impurities beforedissolving silicon into indium by reversing the dissolving order. Inthis case, since pellets or powders of the impurities are to be put intrace amounts as compared with that of silicon, it is difficult touniformly distribute the elements of the impurities in the melt as awhole.

[0032] For the reason described above, it is considered that indiummakes it hard to obtain a high reproducibility of doping though indiumitself has an excellent property as a melt that it can hardly beincorporated into silicon crystals and facilitates to obtain siliconcrystals of high qualities. The inventors therefore considered to diluteand then dissolve impurities into liquid indium. However, a diluent tobe used for this purpose must be a substance which can hardly beincorporated into grown silicon crystals or produces no adverseinfluence even when it is incorporated into the silicon crystals.

[0033] Indium can be used as a first example of adequate diluent. Analloy prepared by dissolving impurities into indium at a predeterminedconcentration makes it possible to more accurately controlconcentrations of the impurities and prevent adverse influences frombeing produced by a diluent incorporated into silicon. Further, such analloy is advantageous also from a viewpoint of having a slight differentdensity from that of a solvent, that is, liquid indium. When the dilutedimpurities are dissolved before dissolving silicon, it is possible toprevent the influence due to a concentration of silicon and use pelletsor powders in large amounts, thereby uniformly dissolving the elementsof the impurities into the melt.

[0034] Silicon can be used as a second example of adequate diluent. Whenthe elements of the impurities are preliminarily diluted with silicon,the elements of the impurities are always and simultaneously dissolvedinto indium with silicon, thereby facilitating to maintainconcentrations of the elements constant relative to that of silicon.

[0035] The present invention which has been achieved on the ideadescribed above will be described in details below with reference toeffects and preferred embodiments thereof. However, the presentinvention is not limited to the following examples.

EXAMPLE 1

[0036] In Example 1, a solar cell having a structure shown in FIG. 1 wasproduced using a metal grade silicon substrate which had a low purityand was inexpensive due to the low purity.

[0037] Meant by the metal grade silicon is silicon which has a purity onthe order of 99% and is obtained by metallurgically reducing silica. Asubstrate 101 of a metal grade polycrystalline silicon which was 0.1 mmthick and 4 inches in diameter was produced by dissolving a metal gradesilicon nugget and gradually cooling it in a carbon die coated withsilicon nitride. The substrate 101 contained boron at a highconcentration and was of a strong p-type. Using a liquid phase growingapparatus which had a configuration shown in FIG. 3, a layer 102 of ap-type polycrystalline silicon was grown on the substrate 101.

[0038] In the apparatus shown in FIG. 3, a crucible 301 made of quartzglass is filled with a dissolved indium melt 302. The apparatus isaccommodated as a whole in a quartz bell-jar 303 and heated to a desiredtemperature from outside with electric furnaces 304. Hydrogen gas isalways introduced into the quartz bell-jar 303 to maintain a reducingatmosphere in the bell-jar 303. Further, a reference numeral 305represents a substrate susceptor made of quartz glass which holds endsof a substrate 300 of a highly pure polycrystalline silicon or thesubstrate 101 of the metal grade polycrystalline silicon 101 having adiameter of 4 inches. The substrate 100 or 300 of the polycrystallinesilicon is held obliquely so as to go and come smoothly into and out ofthe melt 302. A reference numeral 306 designates a load lock chamberwhich can be partitioned from the quartz bell-jar 303 with a gate valve307. When setting silicon in the susceptor 305 or replacing silicon withanother, the susceptor 305 is hoisted up with a hoist mechanism 308 andthe gate valve 307 is closed to prevent an interior of the quartzbell-jar 303 from being exposed to atmosphere. A reference numeral 309represents a dopant introducer which is configured also as a load lockmechanism and allows pellets 310 containing a dopant to be put into theindium melt 302 in a condition where the gate valve 307 is opened andthe susceptor 305 is hoisted up.

[0039] Now, the method of growing the layer 102 of the p-typepolycrystalline silicon will be described concretely. First, the indiummelt 302 was heated to 1000° C. and pellets 310 of highly pure indiumcontaining 1% by weight of aluminum were put into the indium melt 302.Since the indium pellets had a density which was nearly the same as thatof indium, it was considered that the indium pellets were to uniformlydisperse in the melt. Then, the substrate 300 of highly purepolycrystalline silicon was submerged as shown in FIG. 3. The substrate300 was maintained in this condition for 30 minutes to dissolve siliconinto the indium melt 302 until it is saturated.

[0040] Then, the gate valve 307 was closed, the substrate 300 of thehighly pure polycrystalline silicon was removed from the susceptor 305,and a substrate 101 of metal grade polycrystalline silicon having thediameter of 4 inches was placed in the susceptor. After replacing aninternal gas of the load lock chamber 306 first with nitrogen and thenwith hydrogen, the gate valve 307 was opened and the susceptor 305 washoisted down to a preheating position (not shown in the drawings) overthe melt 302 to wait for temperature rise of the substrate 101.Thereafter, cooling of the interior of the quartz bell-jar was startedat a rate of 1° C./minute. When temperature reached 990° C., thesubstrate 101 was submerged into the melt 302. Thirty minutes later, thesusceptor 305 was hoisted up and the load lock chamber 306 was closedwith the gate valve 307. After replacing an internal gas of the loadlock chamber 306 with nitrogen, the substrate 101 was taken outside. Ap-type polycrystalline silicone layer 102 having a thickness of 30 μmhad been grown on the substrate 101.

[0041] A PSG layer (phosphor silicate glass layer) having a thickness of200 Å was deposited on the surface of the p-type polycrystalline siliconlayer 102 at a temperature of 560° C. using a CVD apparatus (not shownin the drawings). An n⁺-type silicon layer 103 was formed on the surfaceside by annealing the PSG layer in a nitrogen gas flow at a temperatureof 1050° C. for 30 minutes and diffusing phosphorus (P). The remainingPSG was eliminated by etching with an aqueous solution of hydrofluoricacid. Furthermore, aluminum was deposited to a thickness of 2 μm on thesurface of the n⁺-type silicon layer 103 by sputtering and comb-teethlike grid electrodes 104 were formed by photolithography. Successively,a titanium oxide film having a thickness of 600 Å was deposited bysputtering as an antireflection film 105. At this stage, pads of thegrid electrodes 104 were masked to prevent titanium oxide from beingdeposited thereon. A solar cell produced as described above willhereinafter referred to as a solar cell 1.

[0042] The characteristic of the solar cell 1 was evaluated with anAM-1.5 solar simulator to obtain a photoelectric conversion efficiencyof 13%. Furthermore, 21 subcells each having an area of 1 cm² wereformed on the substrate 101 and checked for a distribution of thephotoelectric conversion efficiency. The result indicated a distributionwithin ±2% which was a favorable result. Moreover, a silicon crystal wasgrown successively five times while replenishing aluminum and silicon inthe same procedures as those for the first growth in the amount ofaluminum and silicon lost in each growth due to the deposition from themelt. This experiment indicated the variation of the photoelectricconversion efficiency within ±3% at one and the same location of eachsubstrate, which was a favorable result.

[0043] As a comparative example, a solar cell 2 was produced in the sameprocedures as those for the solar cell 1, except that pellets of purealuminum were used as the pellets 310 containing the dopant. In thiscase, aluminum could hardly be incorporated into a p-typepolycrystalline silicon layer 102 even when a dopant was replenished ina theoretically adequate amount. When the dopant was replenished in anamount exceeding the adequate amount, however, irregular spots weregenerated on the surface of the substrate 101 and the p-typepolycrystalline silicon layer 102, which were considered to be formed byreaction between silicon and aluminum. It is presumed that a layer ofmelted aluminum was formed on a surface of the melt and reacted with thesubstrate 101 or the silicon layer 102. The solar cell 2 had remarkablyununiform photoelectric conversion efficiencies which were and certainsubcells exhibited no photoelectric conversion efficiencies at all.Thus, the effects of the present invention was clarified by thiscomparison.

EXAMPLE 2

[0044] Example 2 shows a principle of a method of producing alight-weight and highly efficient solar cell at a low cost by repeatedlyusing an expensive silicon wafer in steps shown in FIGS. 2A to 2C.First, a porous layer 202 which was 5 μm thick was formed on a surfaceof a p⁺-type (100) single crystalline silicon wafer 201 having adiameter of 2 inches by the so-called anodization which applies apositive voltage in hydrofluoric acid. The porous layer is composed of alarge number of micropores having a diameter of 100 Å which are formedby ununiformly dissolving silicon due to electrochemical action ofhydrofluoric acid and extend in a direction of a film thickness whilecomplicatedly tangling with one another. It is possible to epitaxiallygrow a single crystalline silicon on this layer since a portionremaining as a skeleton maintains a property of a single crystal.Methods of forming a porous layer and application of the porous layer tosolar cells are detailed by Japanese Patent Application Laid-Open Nos.5-283722 and 7-302889.

[0045]FIG. 4 shows an apparatus for growing single crystalline siliconwhich was used in Example 2. Reference numerals 401 and 402 representmembers which compose a carbon boat. The member 401 is provided with acavity for dropping substrates 403, 403 a and 403 b for dissolving and acavity for dropping a substrate 404 for growing. The member 402 isprovided with a hole in which an indium melt 405 is to be accommodated.The members 401 and 402 are configured to slide relative to each other.

[0046] A polycrystalline silicon substrate 403 for dissolving siliconinto a melt and a single crystalline silicon substrate 404 having aporous layer 202 formed on the surface for growing a crystal werearranged in the member 401. The member 402 was laid on the member 401and a predetermined amount of highly pure indium pellets were placed inthe hole of the member 402. When the indium pellets were heated in ahydrogen flow, they were melted into a melt 405 as shown in FIG. 4.After maintaining the growing apparatus at 1050° C. for five minutes,the temperature was adjusted to 1000° C. and the melt 405 was broughtinto contact with the substrate 403 for dissolving by sliding the member402. As the substrate 403 for dissolving, a p-type polycrystallinesilicon substrate doped with boron having specific resistance of 0.01Ωcm was used. After keeping this state for one hour, cooling of theapparatus as a whole was started at a rate of 1° C./minute. Whentemperature reached 980° C., the member 402 was slid to bring the melt405 into contact with a surface of the porous layer 202 and cooled forone minute to form a p⁺-type silicon layer 203 having a thickness ofapproximately 1 μm. Thereafter, the melt was returned to its initialposition by sliding the member 402 once again and left standing forcooling.

[0047] When the apparatus was cooled to room temperature, the hardenedmelt and the substrate 403 for dissolving were removed, whereafterhighly pure indium pellets and the substrate 403 a for dissolving madeof the p-type polycrystalline silicon doped with boron and havingspecific resistance of 1 Ωcm were newly arranged and heated in a mannersimilar to that at the preceding stage. After bringing the melt 405 intocontact with the substrate 403 a for dissolving at a temperature of1000° C., keeping it in this condition for one hour, cooling of theapparatus as a whole was started at a rate of 1° C./minute. Whentemperature was lowered to 980° C., the melt 405 was brought intocontact with the surface of the p⁺-type silicon layer 203 by sliding themember 402 once again and cooled for thirty minutes, thereby forming ap-type silicon layer 204 which was approximately 30 μm thick. Then, themelt was returned to its initial position by sliding the member 402 onceagain and left standing for cooling.

[0048] When the melt was cooled to room temperature, the hardened meltand the substrate 403 a for dissolving were removed, whereafter highlypure indium pellets, and a substrate 403 a for dissolving made of n-typepolycrystalline silicon doped with phosphorus and having specificresistance of 0.01 Ωcm were newly disposed and heated in a mannersimilar to that at the preceding stage. After bringing the melt 405 intocontact with the substrate 403 b for dissolving at a temperature of1000° C. and keeping it in this condition for one hour, cooling of theapparatus as a whole was started at a rate of 1° C./minute. Whentemperature was lowered to 980° C., the melt 405 was brought intocontact with the surface of the p-type silicon layer 204 by sliding themember 402 and cooled for thirty seconds, thereby forming an n⁺-typesilicon layer 205 which was approximately 0.5 μm thick. Thereafter, themelt was returned to its initial position by sliding the member 402 onceagain and left standing for cooling.

[0049] Furthermore, aluminum was deposited to form 2 μm thick layer onthe n⁺-type silicon layer 205 by sputtering while masking the layer 205,thereby forming grid electrodes 206. A titanium dioxide film 207 havinga thickness of 600 Å and a magnesium fluoride film 208 having athickness of 1000 Å were stacked as antireflection layers 207 and 208 bysputtering. In sputtering of the antireflection layers, grid tabs weremasked so that the antireflection layers were not deposited thereon.

[0050] A transparent adhesive tape 209 was bonded to a surface of theantireflection layer thus formed. After a stacked body from the p⁺-typesilicon layer 203 to the antireflection layer 208 was peeled from thesilicon wafer 201 by destroying the porous layer 202 by applying forcesin directions indicated by arrows in FIG. 2B, an aluminum sheet 210 wasbonded to a back surface of the p⁺-type silicon layer 203 with anelectroconductive adhesive, thereby forming a solar cell 3.

[0051] The characteristic of the solar cell 3 was evaluated with anAM-1.5 solar simulator to obtain a photoelectric conversion efficiencyof 18%. Furthermore, 26 subcells each having an area of 0.25 cm² wereformed on a substrate 210 of an aluminum sheet and checked for adistribution of photoelectric conversion efficiencies. This resultindicated a distribution within ±3%, which was a favorable result.

[0052] As a comparative example, a solar cell 4 was produced in the sameprocedures as in the case of the solar cell 3, except that a melt wasprepared by arranging powders of boron and phosphorus as dopants in thehole of the member 402 together with indium pellets and that a non-dopedpolycrystalline silicon was used as the silicon for dissolving. Possiblydue to a fact that boron and phosphorus were not uniformly distributedin the melt in the liquid phase growth, photoelectric conversionefficiencies of subcells were 10% at most and distributed within a broadrange, and certain subcells exhibit no photoelectric conversionefficiency at all.

EXAMPLE 3

[0053] Example 3 shows steps for mass production of solar cells having astructure which is basically the same as that of the solar cell producedin Example 2 and proves that the method of the present invention ispreferably applicable to mass production.

[0054] A porous layer 202 having a thickness of 2 μm were formed on each6-inch silicon wafer 201. In this case, the porous layers 202 could beformed on each of the wafers at a time and a working efficiency could beremarkably enhanced by connecting ten silicon wafers 210 in series in asolution of hydrofluoric acid and supplying a current to the wafers.

[0055] An apparatus for growing silicon crystal according to the presentinvention was based on the same principle as that of the apparatusadopted for Example 1 shown in FIG. 3, provided that a substratesusceptor 505 was used which is made of quartz glass and configured tobe capable of accommodating ten substrates. Quartz glass crucibles 501and quartz bell-jars (not shown in the drawings) are deepenedcorrespondingly. The apparatus can be configured so as to accommodate alarger number of substrates to enhance a production efficiency. Threequartz bell-jars having similar internal structures are connected to acommon load lock chamber by way of gate valves so that substrates canmove from one bell-jar into another without being exposed to atmosphere.

[0056] First, a melt was prepared by placing highly pure indium pelletsin a crucible 501 of a first quartz bell-jar, heating and melting thepellets at 1000° C. Highly pure indium pellets containing 1% by weightof aluminum were put into the melt, and then a polycrystalline siliconsubstrate for dissolving was submerged into the melt and kept in thiscondition for 30 minutes to dissolve silicon into the indium melt untilit was saturated, thereby preparing a melt for growing a p⁺-type siliconlayer.

[0057] Then, a melt was prepared by placing highly pure indium pelletsin a crucible of a second quartz bell-jar, heating and melting thepellets at 1000° C. Then, ten substrates of polycrystalline silicondoped with boron and having a specific resistance of 0.05 Ωcm wereattached to a susceptor 505, submerged into the indium melt, kept inthis condition for 30 minutes to dissolve silicon until the indium meltwas saturated, thereby preparing a melt for growing a p-type siliconlayer.

[0058] Further, a melt was prepared by placing highly pure indiumpellets in a crucible of a third quartz ball-jar, heating and meltingthe pellets at 1000° C. Highly pure indium pellets containing 1% byweight of arsenic were put into the melt, and a polycrystalline siliconsubstrate for dissolving was submerged into the melt and kept in thiscondition for 30 minutes to dissolve silicon into the indium melt untilit was saturated, thereby preparing a melt for growing an n⁺-typesilicon layer.

[0059] With the gate valves kept closed, the polycrystalline siliconesubstrate for dissolving was removed from the susceptor 505 and asilicon wafer 201 (hereinafter simply referred to “substrate”) having adiameter of six inches and a porous layer 202 formed on a surfacethereof was set in the susceptor. After replacing an internal gas of theload lock chamber first with nitrogen and then with hydrogen, the gatevalve of the first quartz bell-jar was opened, the susceptor 505 washoisted down to its preheating position, an interior of the quartzbell-jar was maintained at 1050° C. for ten minutes and then cooled to1000° C., and gradual cooling of the interior of the quartz bell-jar wasstarted at a rate of 0.2° C./minute. When temperature reached 995° C.,the substrate was submerged into the melt 502 as shown in FIG. 5. Afterkeeping this condition for ten minutes, the susceptor 505 was hoistedup. A p⁺-type silicon layer 203 having a thickness of approximately 2 μmwas grown on the porous layer 202. Since this apparatus treated a largenumber of substrates and required a time for pulling the susceptor intoand out of the melt, a crystalline silicon growing rate was set at a lowlevel in order not to vary the thickness of the p⁺-type silicon layer203 of each substrate.

[0060] After completely hoisting up the susceptor, the first quartzbell-jar was closed to maintain a hydrogen atmosphere in the load lockchamber, the gate valve of the second bell-jar was opened, the susceptor505 was hoisted down to its preheating position and an interior of thebell-jar was maintained at 1000° C. for ten minutes. Then, gradualcooling of the interior of the quartz bell-jar was started at a rate of1° C./minute. When temperature reached 980° C., the substrate wassubmerged into the melt 502 as shown in FIG. 5. After keeping thiscondition for 30 minutes, the susceptor 505 was hoisted up and the loadlock chamber was closed. A p-type silicon layer 204 having a thicknessof approximately 30 μm was grown on the p⁺-type silicon layer 203.

[0061] While keeping the hydrogen atmosphere in the load lock chamber,the gate valve of the third quartz bell-jar was opened, the susceptor505 was hoisted down to its preheating position, an interior of thequartz bell-jar was maintained at 1000° C. for ten minutes and thengradual cooling was started at a rate of 0.2° C./minute. Whentemperature reached 995° C., the substrate was submerged into the melt502. After maintaining this condition for two minutes, the susceptor 505was hoisted up and the load lock chamber was closed. An n⁺-type siliconlayer 205 having a thickness of approximately 0.4 μm was grown on thep-type silicon layer 204.

[0062] Thereafter, comb-teeth like grid electrodes 206 were formed onthe surface of the n⁺-type silicon layer 205 by printing a copper pasteby the screen printing method and calcining the paste. Successively, atitanium dioxide film 207 having a thickness of 600 Å was formed bycoating a metal alkoxide solution by the sol-gel method and calciningthe solution, and a film (208) of silicon oxide 800 Å thick was formedin the similar procedures as two antireflection layers 207 and 208. Tenor more substrates can easily be treated at a time by the screenprinting method and the sol-gel method which are capable of treating alarge number of substrates. These methods are preferable. Successively,an adhesive tape 209 was bonded to a surface of the antireflectionlayer, the layers of the p⁺-type silicon layer 203 from the upper layerswere peeled from the substrate 201 by applying a force to the substrate201 so as to destroy the porous layer 202, and the tape 209 was peeledoff with an organic solvent. Thereafter, a back surface of the p⁺-typesilicon layer 203 was coated with an electroconductive ink, bonded to analuminum support plate 210 and calcined for setting, thereby producingsolar cells 5.

[0063] Ten solar cells 5 were evaluated with an AM-1.5 solar simulatorto obtain photoelectric conversion efficiencies of 17±0.3%, which werefavorable and uniform. Furthermore, a solar cell module 1 was producedby connecting the ten solar cells in series and bonding them to aheat-resistant glass plate having a thickness of 3 mm with a PVC resin.This solar cell module 1 had an output of approximately 30 W.

[0064] Successively to the module 1, a module 2 was produced in similarprocedures. During the producing, the melts were not cooled but kept inmelted conditions. However, the polycrystalline silicon substrate fordissolving was submerged again into the melt in each of the quartzbell-jars to dissolve silicon until the melt was saturated since siliconconcentration was lowered by deposition of a silicon crystal on thesubstrate. Boron was supplied together with silicon into the melt in thesecond quartz bell-jar. Since dopant concentrations were lowered in themelts in the first and third quartz bell-jars, pellets containing apredetermined amount of aluminum or arsenic were replenished into themelts in the first and third quartz bell-jars before replenishingsilicon. The method of the present invention is capable of uniformlysupplying a dopant with a high repeatability even when using a largecrucible for mass production, whereby the module 2 also exhibited acharacteristic equalled to that of the module 1.

[0065] As a comparative example, ten solar cells were produced at abatch by replenishing the melts with pellets or powders each containinga single element of aluminum, boron or phosphorus. These solar cellsexhibited remarkably variable characteristics, and therefore a solarcell module 3 composed of these solar cells in series had an outputcharacteristic of 5 W, clarifying that the method of the presentinvention is extremely excellent in mass production of modulesconnecting in series.

EXAMPLE 4

[0066] Example 4 shows an example that the method of the presentinvention was applied to the production of a thin film transistor (TFT)of polycrystalline silicon formed on a glass plate which was to be usedin a driving circuit for a liquid crystal display device. FIGS. 6A to 6Fschematically show production steps. A stacked films ofaluminum/chromium having a thickness of 2000 Å were deposited on a glasssubstrate 601 having a size of 4-inch square by sputtering. A patternwas formed as a gate electrode 602 on these films by photolithography(see FIG. 6A). Using disilane and ammonia as raw material gases, asilicon nitride film having a thickness of 3000 Å was deposited as agate insulating film 603 on the gate electrode 602 by the CVD method(see FIG. 6B).

[0067] Used in Example 4 was a growing apparatus having a structurewhich was similar to that of the apparatus shown in FIG. 3 except thattwo quartz bell-jars were connected to a common load lock chamber by wayof gate valves. First, an n-type polycrystalline silicon substrate dopedwith arsenic having a specific resistance of 0.5 Ωcm and a size of4-inch square was submerged into an indium melt 302 in a crucible of afirst quartz bell-jar as shown in FIG. 3 and maintained in thiscondition for thirty minutes to dissolve silicon into the melt 302 untilit was saturated, thereby preparing a melt for an n-type silicon layerfor dissolving. After indium pellets containing 2% by weight of boronwere dropped in a predetermined amount into a highly pure indium melt ina second quartz bell-jar, a highly pure polycrystalline siliconsubstrate having a size of 4-inch square was dissolved thereto, therebypreparing a melt for a p⁺-type silicon layer for dissolving.

[0068] Then, the gate valve was closed, the polycrystalline siliconsubstrate was dismounted from a susceptor, and the glass substrate 601on which the gate insulating film 603 had been formed was set in thesusceptor. After an internal gas of the load lock chamber was replacedwith nitrogen and then with hydrogen, the gate valve of the first quartzbell-jar was opened, and the susceptor was hoisted down to itspreheating position and held at 600° C. for ten minutes to wait fortemperature rise of the substrate 601. Thereafter, gradual cooling of aninterior of the quartz bell-jar was started at a rate of 0.2° C./minute.When temperature reached 595° C., the substrate 601 was submerged intothe melt. The substrate was maintained in this condition for 30 minutesuntil an n-type polycrystalline silicon layer 604 having a thickness of3000 Å was grown on the gate insulating film 603. Then, the susceptorwas hoisted up, the gate valve was closed, the gate valve of the secondquartz bell-jar was opened while maintaining the hydrogen atmosphere,and the susceptor was hoisted down to its preheating position and keptat 600° C. for ten minutes, whereafter gradual cooling of an interior ofthe quartz bell-jar was started at a rate of 0.2° C./minute. Whentemperature reached 595° C., the substrate 601 was submerged in the meltand maintained in this condition for five minutes, whereby a p⁺-typesilicon layer 605 having a thickness of 500 Å was grown on the n-typesilicon layer 604 (see FIG. 6C). Though silicon was grown at a very lowrate in Example 4 due to the use of the glass substrate which did notallow the melt to be heated to a high temperature, the growth could becompleted in a time within a range similar to that for the otherexamples since a necessary layer thickness was small.

[0069] After depositing stacked films of chromium/aluminum bysputtering, a source electrode 606 and a drain electrode 607 werepatterned by photolithography (see FIG. 6D). Using the electrodes 606and 607 as masks, unnecessary portions of the p⁺-type layer at a channelportion 608 were removed by dry etching (see FIG. 6E). Furthermore, asilicon oxide layer 609 was deposited on the surface by sputtering forsurface protection (see FIG. 6F).

[0070] In order to check the TFT for its basic characteristic, −5 V and0 V were applied to the gate electrode while applying 5 V across thesource and drain electrodes. This result indicated an on/off ratio of10⁶. Moreover, a distribution of on/off ratios of b 10 ⁴ TFTs formed inthe substrate was within an extremely narrow range of 120%. Accordingly,it is possible to obtain display devices having a high contrast and freefrom ununiformity in colors by producing a driving circuit of TFTsaccording to the method of the present invention.

[0071] As a comparative example, a dopant was supplied as pellets orpowders each singly composed of a dopant element. In this comparativeexample, on/off ratios of TFTs were distributed within a wide range of10², whereby the TFTs could not be expected to be usable for drivingdisplay devices.

[0072] As understood from the foregoing description, the method of thepresent invention is capable of growing silicon crystals of a highquality having a dopant concentration favorably controlled, therebymaking it possible to produce high performance solar cells, drivingcircuits for liquid crystal display devices and so on at a low cost andwith a high reproducibility.

What is claimed is:
 1. A method of growing a silicon crystal in a liquidphase, which comprises using a melt prepared by dissolving a solid ofsilicon containing a dopant at a predetermined concentration into liquidindium.
 2. A method of growing a silicon crystal in a liquid phaseaccording to claim 1 , wherein the dopant is boron or aluminum.
 3. Amethod of growing a silicon crystal in a liquid phase according to claim1 , wherein the dopant is phosphorus or arsenic.
 4. A method of growinga silicon crystal in a liquid phase, which comprises using a meltprepared by dissolving a solid of indium containing a dopant at apredetermined concentration into liquid indium.
 5. A method of growing asilicon crystal in a liquid phase according to claim 4 , furthercomprising using a melt prepared by further dissolving silicon into themelt in which the dopant is dissolved.
 6. A method of growing a siliconcrystal in a liquid phase according to claim 4 , wherein the dopant isboron or aluminum.
 7. A method of growing a silicon crystal in a liquidphase according to claim 5 , wherein the dopant is boron or aluminum. 8.A method of growing a silicon crystal in a liquid phase according toclaim 4 , wherein the dopant is phosphorus or arsenic.
 9. A method ofgrowing a silicon crystal in a liquid phase according to claim 5 ,wherein the dopant is phosphorus or arsenic.
 10. A method of producing asolar cell, which comprises the steps of: preparing a melt by dissolvinga solid of silicon containing a dopant at a predetermined concentrationinto liquid indium; forming a first silicon layer of a firstconductivity type on a substrate by bringing the substrate into contactwith the melt; and forming a second silicon layer of a secondconductivity type on the first silicon layer of the first conductivitytype.
 11. A method of producing a solar cell according to claim 10 ,wherein the substrate is a silicon wafer which has a porous layer formedon a surface thereof by anodization.
 12. A method of producing a solarcell according to claim 11 , further comprising a step of separating thesilicon wafer from the first silicon layer of the first conductivitytype in the porous layer after forming the second silicon layer of thesecond conductivity type.
 13. A method of producing a solar cellaccording to claim 12 , wherein the separating step is carried out byusing an adhesive tape.
 14. A method of producing a solar cell, whichcomprises the steps of: preparing a melt by dissolving a solid of indiumcontaining a dopant at a predetermined concentration into liquid indiumand then further dissolving silicon into the liquid indium; forming afirst silicon layer of a first conductivity type on a substrate bybringing the substrate into contact with the melt; and forming a secondsilicon layer of a second conductivity type on the first silicon layerof the first conductivity type.
 15. A method of producing a solar cellaccording to claim 14 , wherein the substrate is a silicon wafer whichhas a porous layer formed on a surface thereof by anodization.
 16. Amethod of producing a solar cell according to claim 15 , furthercomprising a step of separating the silicon wafer from the first siliconlayer of the first conductivity type in the porous layer after formingthe second silicon layer of the second conductivity type.
 17. A methodof producing a solar cell according to claim 16 , wherein the separatingstep is carried out by using an adhesive tape.